1. Field of the Invention
The present invention relates to a method and device for locating a nerve. More particularly, the present invention is a method and device for locating the carvernosal nerve having means for determining the stability of a tumescence signal to prevent misinterpretation of a response to application of an electro-stimulus.
2. Related Art
The technique of applying an electro-stimulus to an area of tissue to locate a nerve for the purpose of administering anesthesia or to avoid severing a nerve during the sectioning or excision of tissue has been practiced for decades. Conventional devices for applying an electro-stimulus to an area of tissue have taken the form of an insulated hypodermic needle coupled to an electrical current. In an attempt to locate the nerve, the needle was placed within a tissue site believed to contain the nerve and a pulse of electrical current was applied to the tissue. The effectiveness of each pulse was established by visually inspecting the associated organ or muscle for a response or by taking a report of paresthesia offered by the patient. After evaluating the effectiveness of a pulse of electrical current, the operating physician repositioned the needle and modified the intensity of the next pulse to be applied to the tissue based on his evaluation of the response to the previous pulse. The steps were continued until the operating physician believed the needle to be proximate to the nerve to be located (typically, when a low intensity pulse evoked a strong, immediate response in the associated organ or muscle). While such a device was effective at applying a pulse of electrical current to evoke some type of response, actual localization of the nerve using the afore-described technique was slow and imprecise because the success of the technique was totally dependent on the skill of the operating physician who was responsible for performing each one of the steps.
Localization of a nerve by conventional electro-stimulation is particularly complicated if the nerve comprises multiple, microscopic branches or if the nerve is disposed in a region of the body difficult to reach in light of the surrounding anatomy. Still other nerves, such as autonomic nerves, can be difficult to locate due to the fact that such nerves evoke response patterns which cannot be immediately observed or interpreted by the operating physician administering the electro-stimulus to the tissue site. For example, stimulation of the carvernosal nerve evokes a multi-stage response comprising 1) relaxation of the smooth muscles of the arterioles supplying the penis, 2) dilation of the arteries leading to the penis, 3) constriction of the veins carrying blood away from the penis, and 4) accumulation of blood in the cavernosa. Such a multi-stage response is especially difficult to interpret because the response might not occur until some time after application of the stimulus, perhaps as long as two or more seconds. Given the afore-described factors, it is difficult for an operating physician to determine the location of a nerve via observation of the response pattern alone.
In an effort to reduce the role of the operating physician, and to accommodate the afore-described factors associated with complex or autonomic nerves, attempts have been made to fully automate a nerve locating device. A partially automated device specifically structured to assist in locating the carvernosal nerve is disclosed and claimed in U.S. Pat. Nos. 5,284,153 and 5,284,154 to Raymond et al. The device comprises a stimulating probe, a response detecting means for detecting and measuring tumescence, and a control means comprising means for automatically modulating the intensity of a stimulus to be applied to the tissue believed to contain the carvernosal nerve. With the device, the operating physician positions the stimulating probe beneath the tissue to be stimulated. A stimulus of a pre-determined intensity is applied to the tissue and the response detecting means detects and measures a tumescence response. The stimulating probe is re-positioned by the operating physician and the intensity of the next stimulus to be applied to the tissue is modulated by the automatic modulating means based on an evaluation of the intensity of the tumescence response by the control means. The steps are repeated until the automatic stimulus modulating means of the device converges to a stimulus intensity known to successfully stimulate the carvernosal nerve when the stimulating probe is within 0.5 mm of the nerve.
In an attempt to reduce reliance on the skill of the surgeon, Raymond et al. developed a closed-loop system for locating a nerve, particularly the carvernosal nerve. U.S. Pat. No. 5,775,331 discloses an apparatus and method for locating the carvernosal nerve comprising a stimulating probe having an electrode array, an automatic control means, and a tumescence response detecting means. With the device of U.S. Pat. No. 5,775,131 a stimulus is applied to a target area of tissue by the electrode array of the stimulating probe. The response detecting means records the tumescence response and the control means automatically modifies the stimulus application site and the intensity of the next stimulus to be applied to the target tissue area based on the evaluation of the tumescence response. The method is repeated until the nerve is located.
Through use of the device of U.S. Pat. No. 5,775,331, it has been discovered that other aspects of the nerve locating technique or the surgical method can induce detectable changes in a tumescence signal which can reduce the accuracy of the device. For the carvernosal nerve, for example, it has been determined that physical manipulation of the tumescence monitor, a change in the patient's blood pressure, loss of blood during surgery, the level and type of anesthesia, or natural changes in penile tumescence can evoke a change in the tumescence signal which interferes with the device's ability to accurately locate the nerve. Furthermore, the response to stimulation for some visceral or autonomic nerves (such as the carvernosal nerve) may persist for several seconds after cessation of successful stimulation and, in some cases, refractory responses may follow an initial response to successful stimulation.
Thus, there is a need for a device for locating a nerve which is closed-loop (and, thus, independent of the skill of the operator) and comprises means for determining if a signal from a nerve fiber, organ or muscle is stable prior to application of an electro-stimulus to avoid misinterpretation of a response to electro-stimulation.